1. Field of the Invention
The present invention relates in general to a marine propulsion system, and in particular to a transmission for a propulsion system of an outboard drive.
2. Description of Related Art
Many outboard drives of marine watercrafts employ forward/neutral/reverse transmissions together with a dual propeller propulsion system. Such transmissions are common in both outboard motors and in outboard drive units of inboard/outboard motors.
These transmissions typically include a driving bevel gear and a pair of oppositely rotating driven bevel gears. Each driven gear includes a hub that is journaled within a lower unit of the outboard drive. A front dog clutch of a dual clutch assembly is interposed between the pair of oppositely rotating gears. In this position, the front dog clutch moves between positions in which the clutch engages the gears. The front dog clutch selectively couples an inner propeller shaft to one of the driven gears to rotate a rear propeller in either a forward or a reverse direction.
The transmission also includes a second dog clutch that is positioned to the rear side of the rear driven gear hub. The rear clutch selectively engages corresponding teeth formed on the rear side of the hub of the rear gear to drive an outer propeller shaft. The outer propulsion shaft in turn drives a front propeller.
Such prior transmission designs tend to occupy a significant amount of space in the lower unit on the rear side of the drive shaft. The lower unit also houses an exhaust passageway for the discharge of engine exhaust.
The large size of prior transmissions used with counter-rotational propulsion systems commonly leaves less space for the exhaust passage though the lower unit. Inadequate exhaust flow area can result in higher back pressure, and engine exhaust tends not to discharge smoothly. Engine performance consequently suffers. This problem becomes more acute with larger engines. It becomes necessary to increase the flow area of the exhaust passage through the lower unit in order to discharge exhaust gas smoothly.
Lower units thus have increased in size to accommodate the larger exhaust passages with current transmission designs. An increased size in the lower unit, however, undesirably increases the resistance to fluid flow around the lower unit, i.e., undesirably increases the drag on the lower unit.
Another disadvantage associated with prior transmissions used with counter-rotational propulsion systems is that such systems rotate the rear propeller when driving the watercraft in the reverse direction. The front propeller, however, tends to block the thrust stream produced by the rear propeller and thereby inhibits the performance of the outboard drive when operated in reverse.